V2X operation method performed by terminal in wireless communication system and terminal using same method

ABSTRACT

The present invention provides a device-to-device (D2D) operation method performed by a tenninal in a wireless communication system, the method comprising: determining whether a transmission time point of a first message having a high priority overlaps a transmission time point of a second message having a priority lower than that of the first message; and transmitting the second message using transmission power lower than transmission power of the first message on the basis of the determination, wherein the transmission of the second message is configured to repeat a predetermined number of times.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2016/009214, filed on Aug. 19, 2016, which claims the benefit of U.S. Provisional Application No. 62/206,901, filed on Aug. 19, 2015, the contents of which are all hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the invention

The present invention relates to wireless communication, and more particularly, to a V2X operation method performed by a terminal in a wireless communication system and a terminal using the same method.

Related Art

In International Telecommunication Union Radio communication sector (ITU-R), a standardization task for International Mobile Telecommunication (IMT)-Advanced, that is, a next-generation mobile communication system after the 3rd generation, is in progress. IMT-Advanced sets its goal to support Internet Protocol (IP)-based multimedia services at a data transfer rate of 1 Gbps in the stop and slow-speed moving state and of 100 Mbps in the fast-speed moving state.

The 3rd generation partnership project (3GPP) is a system standard to satisfy the requirements of IMT-Advanced and prepares LTE-advanced improved from long term evolution (LTE) based on orthogonal frequency division multiple access (OFDMA)/single carrier-frequency division multiple access (SC-FDMA) transmission schemes. LTE-Advanced is one of strong candidates for IMT-Advanced.

There is a growing interest in a device-to-device (D2D) technology in which devices perform direct communication. In particular, D2D has been in the spotlight as a communication technology for a public safety network. A commercial communication network is rapidly changing to LTE, but the current public safety network is basically based on the 2G technology in terms of a collision problem with existing communication standards and a cost. Such a technology gap and a need for improved services are leading to efforts to improve the public safety network.

The public safety network has higher service requirements (reliability and security) than the commercial communication network. In particular, if coverage of cellular communication is not affected or available, the public safety network also requires direct communication between devices, that is, D2D communication.

A D2D operation may have various advantages in signal transmission/reception between proximity devices. For example, a D2D UE has a high transfer rate and a low delay and may perform data communication. Furthermore, in a D2D operation, traffic concentrated on an eNB can be distributed. If a D2D UE plays the role of a relay, it may also function to extend coverage of an eNB.

In the D2D communication, D2D communication mainly related to a vehicle is called, in particular, vehicle-to-X (V2X) communication. A terminal can perform the V2X communication based on limited power. Accordingly, the terminal can perform transmission based on relatively high power for a signal having a relatively high priority and perform transmission based on relatively low power for a signal having a relatively low priority.

In this case, when the low power is allocated when the signal with the relatively low priority is transmitted, power lower than minimum power required for transmission of the signal with the relatively low priority may be allocated.

Accordingly, the present invention intends to provide a method for solving a problem that, when transmitting the signal with the relatively low priority, the power lower than the minimum power is allocated and an apparatus using the method.

SUMMARY OF THE INVENTION

The present invention provides a V2X operation method performed by a terminal in a wireless communication system and a terminal using the same method.

In an aspect, a method for device-to-device (D2D) operation in a wireless communication system is provided. The method may be performed by a user equipment (UE) and comprise determining whether a transmission time point of a first message having a high priority overlaps a transmission time point of a second message having a priority lower than that of the first message, and transmitting the second message using transmission power lower than transmission power of the first message on the basis of the determination. The transmission of the second message may be configured to repeat a predetermined number of times.

The second message may be transmitted at a number of times greater than the predetermined number of transmission times of the second message when the transmission time point of the first message overlaps with the transmission time point of the second message.

A value of the greater number of times may vary depending on a degree at which a value of power allocated to the transmission of the second message decreases.

The value of the greater number of times may increase as the value of the power allocated to the transmission of the second message decreases.

When the decrease in value of the power allocated to the transmission of the second message is not temporary, differences between the existing value of the power allocated to the transmission of the second message and the value of the power allocated to the transmission of the second message may be accumulated and the value of the greater number of times varies depending on the accumulated values.

The method may further comprise receiving from a base station information indicating whether the second message is allowed to be transmitted at the greater number of times than the predetermined number of times.

The method may further comprise transmitting to the base station information indicating that the second message is transmitted at the greater number of times than the predetermined number of times.

Transmission power having a relatively large value may be used for transmission of the second message, which corresponds to the remaining number of transmission times other than the number of transmission times of the second message using lower transmission power in the predetermined number of times when the transmission time point of the first message overlaps with the transmission time point of the second message.

A predetermined offset may be applied to the transmission of the second message, which corresponds to the remaining number of transmission times.

The transmission of the second message, which corresponds to the remaining number of transmission times, may be performed in a dedicated resource pool.

The first message may be a wide area network (WAN) uplink message and the second message is a D2D message.

The first message may be a D2D synchronization message and the second message is a D2D communication message.

In overlapping of the transmission time point of the first message and the transmission time point of the second message, the transmission time point of the first message and the transmission time point of the second message may overlap with each other in a frequency division multiplexing (FDM) form in one carrier or the transmission time point of the first message and the transmission time point of the second message overlap with each other in different carriers.

The D2D operation may be a vehicle to X (V2X) operation.

In another aspect, a user equipment (UE) is provided. The UE may comprise a radio frequency (RF) unit transmitting and receiving a radio signal and a processor operated in association with the RF unit. The processor may determine whether a transmission time point of a first message having a high priority overlaps a transmission time point of a second message having a priority lower than that of the first message, and transmit the second message using transmission power lower than transmission power of the first message on the basis of the determination. The transmission of the second message may be configured to repeat a predetermined number of times.

According to the present invention, provided are a method for a D2D operation performed by a terminal in a wireless communication system and a terminal using the same method.

According to the present invention, provided are a V2X operation method performed by a terminal in a wireless communication system and a terminal using the same method. In this case, the terminal can perform the V2X communication based on limited power. Accordingly, the terminal can perform transmission based on relatively high power for a signal having a relatively high priority and perform transmission based on relatively low power for a signal having a relatively low priority. Moreover, when the terminal transmits the signal having the relatively low priority, in the case where the lower power than minimum power is allocated to the transmission of the signal with the relatively low priority, a message to which the lower power is allocated is transmitted at the number of times more than a predetermined number of times to compensate reduction of coverage and/or performance of a message having the low priority due to reduction of transmission power by repeated transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the present invention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a user plane.

FIG. 3 is a diagram showing a wireless protocol architecture for a control plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idle state.

FIG. 5 is a flowchart illustrating a procedure of establishing RRC connection.

FIG. 6 is a flowchart illustrating an RRC connection reconfiguration procedure.

FIG. 7 is a diagram illustrating an RRC connection re-establishment procedure.

FIG. 8 illustrates sub states where the terminal may have in an RRC IDLE state and a sub state transition process.

FIG. 9 illustrates a reference structure for a ProSe.

FIG. 10 illustrates arrangement examples of terminals performing ProSe direct communication and cell coverage.

FIG. 11 illustrates a user plane protocol stack for the ProSe direct communication.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

FIG. 13 is a flowchart of a method for performing V2X communication according to an embodiment of the present invention.

FIG. 14 is a flowchart of a method for performing V2X communication according to another embodiment of the present invention.

FIG. 15 is a flowchart of a method for performing V2X communication according to yet another embodiment of the present invention.

FIG. 16 is a block diagram illustrating a UE in which the embodiment of the present invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the present invention is applied. The wireless communication system may also be referred to as an evolved-UMTS terrestrial radio access network (E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides a control plane and a user plane to a user equipment (UE) 10. The UE 10 may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a mobile terminal (MT), a wireless device, etc. The BS 20 is generally a fixed station that communicates with the UE 10 and may be referred to as another terminology, such as an evolved node-B (eNB), a base transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20 are also connected by means of an S1 interface to an evolved packet core (EPC) 30, more specifically, to a mobility management entity (MME) through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway (P-GW). The MME has access information of the UE or capability information of the UE, and such information is generally used for mobility management of the UE. The S-GW is a gateway having an E-UTRAN as an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network can be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on the lower three layers of the open system interconnection (OSI) model that is well-known in the communication system. Among them, a physical (PHY) layer belonging to the first layer provides an information transfer service by using a physical channel, and a radio resource control (RRC) layer belonging to the third layer serves to control a radio resource between the UE and the network. For this, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a user plane. FIG. 3 is a diagram showing a wireless protocol architecture for a control plane. The user plane is a protocol stack for user data transmission. The control plane is a protocol stack for control signal transmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with an information transfer service through a physical channel. The PHY layer is connected to a medium access control (MAC) layer which is an upper layer of the PHY layer through a transport channel. Data is transferred between the MAC layer and the PHY layer through the transport channel. The transport channel is classified according to how and with what characteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of a transmitter and a receiver, through a physical channel. The physical channel may be modulated according to an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and use the time and frequency as radio resources.

The functions of the MAC layer include mapping between a logical channel and a transport channel and multiplexing and demultiplexing to a transport block that is provided through a physical channel on the transport channel of a MAC Service Data Unit (SDU) that belongs to a logical channel. The MAC layer provides service to a Radio Link Control (RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation, and reassembly of an RLC SDU. In order to guarantee various types of Quality of Service (QoS) required by a Radio Bearer (RB), the RLC layer provides three types of operation mode: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provides error correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer is related to the configuration, reconfiguration, and release of radio bearers, and is responsible for control of logical channels, transport channels, and PHY channels. An RB means a logical route that is provided by the first layer (PHY layer) and the second layers (MAC layer, the RLC layer, and the PDCP layer) in order to transfer data between UE and a network.

The function of a Packet Data Convergence Protocol (PDCP) layer on the user plane includes the transfer of user data and header compression and ciphering. The function of the PDCP layer on the user plane further includes the transfer and encryption/integrity protection of control plane data.

What an RB is configured means a procedure of defining the characteristics of a wireless protocol layer and channels in order to provide specific service and configuring each detailed parameter and operating method. An RB can be divided into two types of a Signaling RB (SRB) and a Data RB (DRB). The SRB is used as a passage through which an RRC message is transmitted on the control plane, and the DRB is used as a passage through which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRC layer of an E-UTRAN, the UE is in the RRC connected state. If not, the UE is in the RRC idle state.

A downlink transport channel through which data is transmitted from a network to UE includes a broadcast channel (BCH) through which system information is transmitted and a downlink shared channel (SCH) through which user traffic or control messages are transmitted. Traffic or a control message for downlink multicast or broadcast service may be transmitted through the downlink SCH, or may be transmitted through an additional downlink multicast channel (MCH). Meanwhile, an uplink transport channel through which data is transmitted from UE to a network includes a random access channel (RACH) through which an initial control message is transmitted and an uplink shared channel (SCH) through which user traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that are mapped to the transport channel include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic channel (MTCH).

The physical channel includes several OFDM symbols in the time domain and several subcarriers in the frequency domain. One subframe includes a plurality of OFDM symbols in the time domain. An RB is a resources allocation unit, and includes a plurality of OFDM symbols and a plurality of subcarriers. Furthermore, each subframe may use specific subcarriers of specific OFDM symbols (e.g., the first OFDM symbol) of the corresponding subframe for a physical downlink control channel (PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval (TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logically connected to the RRC layer of the E-UTRAN. A case where the RRC layer of UE is logically connected to the RRC layer of the E-UTRAN is referred to as an RRC connected state. A case where the RRC layer of UE is not logically connected to the RRC layer of the E-UTRAN is referred to as an RRC idle state. The E-UTRAN may check the existence of corresponding UE in the RRC connected state in each cell because the UE has RRC connection, so the UE may be effectively controlled. In contrast, the E-UTRAN is unable to check UE in the RRC idle state, and a Core Network (CN) manages UE in the RRC idle state in each tracking area, that is, the unit of an area greater than a cell. That is, the existence or non-existence of UE in the RRC idle state is checked only for each large area. Accordingly, the UE needs to shift to the RRC connected state in order to be provided with common mobile communication service, such as voice or data.

When a user first powers UE, the UE first searches for a proper cell and remains in the RRC idle state in the corresponding cell. The UE in the RRC idle state establishes RRC connection with an E-UTRAN through an RRC connection procedure when it is necessary to set up the RRC connection, and shifts to the RRC connected state. A case where UE in the RRC idle state needs to set up RRC connection includes several cases. For example, the cases may include a need to send uplink data for a reason, such as a call attempt by a user, and to send a response message as a response to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performs functions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types of states: EPS Mobility Management-REGISTERED (EMM-REGISTERED) and EMM-DEREGISTERED are defined. The two states are applied to UE and the MME. UE is initially in the EMM-DEREGISTERED state. In order to access a network, the UE performs a procedure of registering it with the corresponding network through an initial attach procedure. If the attach procedure is successfully performed, the UE and the MME become the EMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, two types of states: an EPS Connection Management (ECM)-IDLE state and an ECM-CONNECTED state are defined. The two states are applied to UE and the MME. When the UE in the ECM-IDLE state establishes RRC connection with the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in the ECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1 connection with the E-UTRAN. When the UE is in the ECM-IDLE state, the E-UTRAN does not have information about the context of the UE. Accordingly, the UE in the ECM-IDLE state performs procedures related to UE-based mobility, such as cell selection or cell reselection, without a need to receive a command from a network. In contrast, when the UE is in the ECM-CONNECTED state, the mobility of the UE is managed in response to a command from a network. If the location of the UE in the ECM-IDLE state is different from a location known to the network, the UE informs the network of its corresponding location through a tracking area update procedure.

System information is described below.

System information includes essential information that needs to be known by UE in order for the UE to access a BS. Accordingly, the UE needs to have received all pieces of system information before accessing the BS, and needs to always have the up-to-date system information. Furthermore, the BS periodically transmits the system information because the system information is information that needs to be known by all UEs within one cell. The system information is divided into a Master Information Block (MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include a limited number of parameters that are most essential and most frequently transmitted when other information is required to be obtained from a cell. UE first searches for an MIB after downlink synchronization. The MIB may include information, such as an SFN that supports downlink channel bandwidth, a PHICH configuration, and synchronization and operates as a timing criterion and an eNB transmit antenna configuration. The MIB may be transmitted on a broadcast channel (BCH) through broadcasting.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a “SystemInformationBlockType1” message and transmitted. The remaining SIBs other than the SIB1 is included in a system information message and transmitted. To map the SIBs to the system information message may be flexibly configured by a scheduling information list parameter included in the SIB1. In this case, each of the SIBs is included in a single system information message, and only SIBs having the same scheduling requirement value (e.g. cycle) may be mapped to the same system information message. Furthermore, a SystemInformationBlockType2 (SIB2) is always mapped to a system information message corresponding to the first entry within the system information message list of a scheduling information list. A plurality of system information messages may be transmitted within the same cycle. The SIB1 and all the system information messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in an E-UTRAN, the SIB1 may be dedicated-signaled in the state in which it includes a parameter configured like an existing configured value. In this case, the SIB1 may be included in an RRC connection reconfiguration message and transmitted.

The SIB1 includes information related to UE cell access, and defines the scheduling of other SIBs. The SIB1 may include information related to the PLMN identifiers of a network, tracking area code (TAC) and a cell ID, a cell barring status indicative of whether a cell is a cell on which camp-on is possible, the lowest reception level required within a cell which is used as cell reselection criterion, and the transmission time and cycle of other SIBs.

The SIB2 may include radio resource configuration information common to all pieces of UE. The SIB2 may include information related to an uplink carrier frequency and uplink channel bandwidth, an RACH configuration, a page configuration, an uplink power control configuration, a sounding reference signal configuration, a PUCCH configuration supporting ACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and detecting a change of system information to a primary cell (PCell) only. In a secondary cell (SCell), when a corresponding SCell is added, an E-UTRAN may provide all of pieces of system information related to an RRC connection state operation through dedicated signaling. When system information related to a configured SCell is changed, an E-UTRAN may release an SCell that is taken into consideration and subsequently add the changed system information. This may be performed along with a single RRC connection reconfiguration message. An E-UTRAN may configure parameter values different from a value broadcasted within an SCell that has been taken into consideration through dedicated signaling.

UE needs to guarantee the validity of a specific type of system information, and such system information is called required system information. The required system information may be defined as follows.

If UE is an RRC idle state: The UE needs to be guaranteed so that it has the valid versions of the MIB and the SIB1 in addition to the SIB2 to SIB8. This may comply with the support of a radio access technology (RAT) that is taken into consideration.

If UE is an RRC connection state: The UE needs to be guaranteed so that it has the valid versions of the MIB, the SIB1, and the SIB2.

In general, the validity of system information may be guaranteed up to a maximum of 3 hours after the system information is obtained.

In general, service that is provided to UE by a network may be classified into three types as follows. Furthermore, the UE differently recognizes the type of cell depending on what service may be provided to the UE. In the following description, a service type is first described, and the type of cell is described.

1) Limited service: this service provides emergency calls and an Earthquake and Tsunami Warning System (ETWS), and may be provided by an acceptable cell.

2) Suitable service: this service means public service for common uses, and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communication network operators. This cell may be used by only communication network operators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell may be classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be provided with limited service. This cell is a cell that has not been barred from a viewpoint of corresponding UE and that satisfies the cell selection criterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be provided with suitable service. This cell satisfies the conditions of an acceptable cell and also satisfies additional conditions. The additional conditions include that the suitable cell needs to belong to a Public Land Mobile Network (PLMN) to which corresponding UE may access and that the suitable cell is a cell on which the execution of a tracking area update procedure by the UE is not barred. If a corresponding cell is a CSG cell, the cell needs to be a cell to which UE may access as a member of the CSG.

3) A barred cell: this cell is a cell that broadcasts information indicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts information indicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idle state. FIG. 4 illustrates a procedure in which UE that is initially powered on experiences a cell selection procedure, registers it with a network, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) in which the UE communicates with a Public Land Mobile Network (PLMN), that is, a network from which the UE is provided with service (S410). Information about the PLMN and the RAT may be selected by the user of the UE, and the information stored in a Universal Subscriber Identity Module (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs to cells having measured BS and signal intensity or quality greater than a specific value (cell selection) (S420). In this case, the UE that is powered off performs cell selection, which may be called initial cell selection. A cell selection procedure is described later in detail. After the cell selection, the UE receives system information periodically by the BS. The specific value refers to a value that is defined in a system in order for the quality of a physical signal in data transmission/reception to be guaranteed. Accordingly, the specific value may differ depending on applied RAT.

If network registration is necessary, the UE performs a network registration procedure (S430). The UE registers its information (e.g., an IMSI) with the network in order to receive service (e.g., paging) from the network. The UE does not register it with a network whenever it selects a cell, but registers it with a network when information about the network (e.g., a Tracking Area Identity (TAI)) included in system information is different from information about the network that is known to the UE.

The UE performs cell reselection based on a service environment provided by the cell or the environment of the UE (S440). If the value of the intensity or quality of a signal measured based on a BS from which the UE is provided with service is lower than that measured based on a BS of a neighboring cell, the UE selects a cell that belongs to other cells and that provides better signal characteristics than the cell of the BS that is accessed by the UE. This procedure is called cell reselection differently from the initial cell selection of the No. 2 procedure. In this case, temporal restriction conditions are placed in order for a cell to be frequently reselected in response to a change of signal characteristic. A cell reselection procedure is described later in detail.

FIG. 5 is a flowchart illustrating a procedure of establishing RRC connection.

UE sends an RRC connection request message that requests RRC connection to a network (S510). The network sends an RRC connection establishment message as a response to the RRC connection request (S520). After receiving the RRC connection establishment message, the UE enters RRC connected mode.

The UE sends an RRC connection establishment complete message used to check the successful completion of the RRC connection to the network (S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfiguration procedure. An RRC connection reconfiguration is used to modify RRC connection. This is used to establish/modify/release RBs, perform handover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifying RRC connection to UE (S610). As a response to the RRC connection reconfiguration message, the UE sends an RRC connection reconfiguration complete message used to check the successful completion of the RRC connection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile network operator. Each mobile network operator operates one or more PLMNs. Each PLMN may be identified by a Mobile Country Code (MCC) and a Mobile Network Code (MNC). PLMN information of a cell is included in system information and broadcasted.

In PLMN selection, cell selection, and cell reselection, various types of PLMNs may be considered by the terminal.

Home PLMN (HPLMN):PLMN having MCC and MNC matching with MCC and MNC of a terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing location registration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a general service is provided to the terminal through the HPLMN or the EHPLMN, the terminal is not in a roaming state. Meanwhile, when the service is provided to the terminal through a PLMN except for the HPLMN/EHPLMN, the terminal is in the roaming state. In this case, the PLMN refers to a Visited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available Public Land Mobile Networks (PLMNs) and selects a proper PLMN from which the UE is able to be provided with service. The PLMN is a network that is deployed or operated by a mobile network operator. Each mobile network operator operates one or more PLMNs. Each PLMN may be identified by Mobile Country Code (MCC) and Mobile Network Code (MNC). Information about the PLMN of a cell is included in system information and broadcasted. The UE attempts to register it with the selected PLMN. If registration is successful, the selected PLMN becomes a Registered PLMN (RPLMN). The network may signalize a PLMN list to the UE. In this case, PLMNs included in the PLMN list may be considered to be PLMNs, such as RPLMNs. The UE registered with the network needs to be able to be always reachable by the network. If the UE is in the ECM-CONNECTED state (identically the RRC connection state), the network recognizes that the UE is being provided with service. If the UE is in the ECM-IDLE state (identically the RRC idle state), however, the situation of the UE is not valid in an eNB, but is stored in the MME. In such a case, only the MME is informed of the location of the UE in the ECM-IDLE state through the granularity of the list of Tracking Areas (TAs). A single TA is identified by a Tracking Area Identity (TAI) formed of the identifier of a PLMN to which the TA belongs and Tracking Area Code (TAC) that uniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by the selected PLMN and that has signal quality and characteristics on which the UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting a cell by a terminal.

When power is turned-on or the terminal is located in a cell, the terminal performs procedures for receiving a service by selecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a service through the cell by always selecting a suitable quality cell. For example, a terminal where power is turned-on just before should select the suitable quality cell to be registered in a network. If the terminal in an RRC connection state enters in an RRC idle state, the terminal should selects a cell for stay in the RRC idle state. In this way, a procedure of selecting a cell satisfying a certain condition by the terminal in order to be in a service idle state such as the RRC idle state refers to cell selection. Since the cell selection is performed in a state that a cell in the RRC idle state is not currently determined, it is important to select the cell as rapid as possible. Accordingly, if the cell provides a wireless signal quality of a predetermined level or greater, although the cell does not provide the best wireless signal quality, the cell may be selected during a cell selection procedure of the terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTE is described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “User Equipment (UE) procedures in idle mode (Release 8)”.

A cell selection procedure is basically divided into two types.

The first is an initial cell selection procedure. In this procedure, UE does not have preliminary information about a wireless channel. Accordingly, the UE searches for all wireless channels in order to find out a proper cell. The UE searches for the strongest cell in each channel. Thereafter, if the UE has only to search for a suitable cell that satisfies a cell selection criterion, the UE selects the corresponding cell.

Next, the UE may select the cell using stored information or using information broadcasted by the cell. Accordingly, cell selection may be fast compared to an initial cell selection procedure. If the UE has only to search for a cell that satisfies the cell selection criterion, the UE selects the corresponding cell. If a suitable cell that satisfies the cell selection criterion is not retrieved though such a procedure, the UE performs an initial cell selection procedure.

A cell selection criterion may be defined as in Equation 1 below. Following Equation 1 can be referred to as measurement for determining whether or not S-criterion is satisfied. Srxlev>0 AND Squal>0. where: Srxlev=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(rxlevminoffset))−P _(compensation), Squal=Q _(qualmeas)(Q _(qualmin) +Q _(qualminoffset))  [Equation 1]

In this case, in Equation 1, the variables may be defined as in Table 1 below.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selection quality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP) Q_(qalmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimum required RX level in the cell (dBm) Q_(qualmin) Minimum required quality level in the cell (dB) Q_(rxlevminoffset) Offset to the signalled Q_(rxlevmin) taken into account in the Srxlev evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken into account in the Squal evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN Pcompensation max(P_(EMAX)-P_(PowerClass,) 0) ) (dB) P_(EMAX) Maximum TX power level an UE may use when transmitting on the uplink in the cell (dBm) defined as P_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of the UE (dBm) according to the UE power class as defined in [TS 36.101]

Qrxlevminoffset and Qqualminoffset, that is, signaled values, are the results of periodic discovery for a PLMN having higher priority while UE camps on a normal cell within a VPLMN, and may be applied only when cell selection is evaluated. As described above, during the periodic discovery of a PLMN having higher priority, UE may perform cell selection evaluation using parameter values stored from another cell of the PLMN having such higher priority.

After UE selects any cell through a cell selection procedure, the intensity or quality of a signal between the UE and a BS may be changed due to the mobility of the UE or a change of a radio environment. Accordingly, if the quality of the selected cell is changed, the UE may select another cell providing better quality.

After the UE selects a specific cell through the cell selection procedure, the intensity or quality of a signal between the UE and a BS may be changed due to a change in the mobility or wireless environment of the UE. Accordingly, if the quality of the selected cell is deteriorated, the UE may select another cell that provides better quality. If a cell is reselected as described above, the UE selects a cell that provides better signal quality than the currently selected cell. Such a procedure is called cell reselection. In general, a basic object of the cell reselection procedure is to select a cell that provides UE with the best quality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a network may determine priority corresponding to each frequency, and may inform the UE of the determined priorities. The UE that has received the priorities preferentially takes into consideration the priorities in a cell reselection procedure compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cell according to the signal characteristics of a wireless environment. In selecting a cell for reselection when a cell is reselected, the following cell reselection methods may be present according to the RAT and frequency characteristics of the cell.

Intra-frequency cell reselection: UE reselects a cell having the same center frequency as that of RAT, such as a cell on which the UE camps on.

Inter-frequency cell reselection: UE reselects a cell having a different center frequency from that of RAT, such as a cell on which the UE camps on

Inter-RAT cell reselection: UE reselects a cell that uses RAT different from RAT on which the UE camps

The principle of a cell reselection procedure is as follows.

First, UE measures the quality of a serving cell and neighbor cells for cell reselection.

Second, cell reselection is performed based on a cell reselection criterion. The cell reselection criterion has the following characteristics in relation to the measurements of a serving cell and neighbor cells.

Intra-frequency cell reselection is basically based on ranking. Ranking is a task for defining a criterion value for evaluating cell reselection and numbering cells using criterion values according to the size of the criterion values. A cell having the best criterion is commonly called the best-ranked cell. The cell criterion value is based on the value of a corresponding cell measured by UE, and may be a value to which a frequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority provided by a network. UE attempts to camp on a frequency having the highest frequency priority. A network may provide frequency priority that will be applied by UEs within a cell in common through broadcasting signaling, or may provide frequency-specific priority to each UE through UE-dedicated signaling. A cell reselection priority provided through broadcast signaling may refer to a common priority. A cell reselection priority for each terminal set by a network may refer to a dedicated priority. If receiving the dedicated priority, the terminal may receive a valid time associated with the dedicated priority together. If receiving the dedicated priority, the terminal starts a validity timer set as the received valid time together therewith. While the valid timer is operated, the terminal applies the dedicated priority in the RRC idle mode. If the valid timer is expired, the terminal discards the dedicated priority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE with a parameter (e.g., a frequency-specific offset) used in cell reselection for each frequency. For the intra-frequency cell reselection or the inter-frequency cell reselection, a network may provide UE with a Neighboring Cell List (NCL) used in cell reselection. The NCL includes a cell-specific parameter (e.g., a cell-specific offset) used in cell reselection. For the intra-frequency or inter-frequency cell reselection, a network may provide UE with a cell reselection black list used in cell reselection.

The UE does not perform cell reselection on a cell included in the black list.

Ranking performed in a cell reselection evaluation procedure is described below.

A ranking criterion used to give the priority of a cell is defined as in Equation 2. R _(s) =Q _(meas,s) +Q _(hyst) ,R _(n) =Q _(meas,n) −Q _(offset)  [Equation 2]

In Equation 2, Rs is the ranking criterion of a serving cell on which UE now camps, Rn is the ranking criterion of a neighboring cell, Qmeas,s is the quality value of the serving cell measured by the UE, Qmeas,n is the quality value of the neighboring cell measured by the UE, Qhyst is a hysteresis value for ranking, and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between a serving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does not Qoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for a corresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does not receive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterion Rn of a neighbor cell are changed in a similar state, ranking priority is frequency changed as a result of the change, and UE may alternately reselect the twos. Qhyst is a parameter that gives hysteresis to cell reselection so that UE is prevented from to alternately reselecting two cells.

UE measures RS of a serving cell and Rn of a neighbor cell according to the above equation, considers a cell having the greatest ranking criterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality of a cell is the most important criterion in cell reselection. If a reselected cell is not a suitable cell, UE excludes a corresponding frequency or a corresponding cell from the subject of cell reselection.

Hereinafter, radio link failure (RLF) will be described.

UE continues to perform measurements in order to maintain the quality of a radio link with a serving cell from which the UE receives service. The UE determines whether or not communication is impossible in a current situation due to the deterioration of the quality of the radio link with the serving cell. If communication is almost impossible because the quality of the serving cell is too low, the UE determines the current situation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication with the current serving cell, selects a new cell through cell selection (or cell reselection) procedure, and attempts RRC connection re-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken as cases where normal communication is impossible.

A case where UE determines that there is a serious problem in the quality of a downlink communication link (a case where the quality of a PCell is determined to be low while performing RLM) based on the radio quality measured results of the PHY layer of the UE

A case where uplink transmission is problematic because a random access procedure continues to fail in the MAC sublayer.

A case where uplink transmission is problematic because uplink data transmission continues to fail in the RLC sublayer.

A case where handover is determined to have failed.

A case where a message received by UE does not pass through an integrity check.

An RRC connection re-establishment procedure is described in more detail below.

FIG. 7 is a diagram illustrating an RRC connection re-establishment procedure.

Referring to FIG. 7, UE stops using all the radio bearers that have been configured other than a Signaling Radio Bearer (SRB) #0, and initializes a variety of kinds of sublayers of an Access Stratum (AS) (S710). Furthermore, the UE configures each sublayer and the PHY layer as a default configuration. In this procedure, the UE maintains the RRC connection state.

The UE performs a cell selection procedure for performing an RRC connection reconfiguration procedure (S720). The cell selection procedure of the RRC connection re-establishment procedure may be performed in the same manner as the cell selection procedure that is performed by the UE in the RRC idle state, although the UE maintains the RRC connection state.

After performing the cell selection procedure, the UE determines whether or not a corresponding cell is a suitable cell by checking the system information of the corresponding cell (S730). If the selected cell is determined to be a suitable E-UTRAN cell, the UE sends an RRC connection re-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RAT different from that of the E-UTRAN through the cell selection procedure for performing the RRC connection re-establishment procedure, the UE stops the RRC connection re-establishment procedure and enters the RRC idle state (S750).

The UE may be implemented to finish checking whether the selected cell is a suitable cell through the cell selection procedure and the reception of the system information of the selected cell. To this end, the UE may drive a timer when the RRC connection re-establishment procedure is started. The timer may be stopped if it is determined that the UE has selected a suitable cell. If the timer expires, the UE may consider that the RRC connection re-establishment procedure has failed, and may enter the RRC idle state. Such a timer is hereinafter called an RLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as an RLF timer. The UE may obtain the set value of the timer from the system information of the serving cell.

If an RRC connection re-establishment request message is received from the UE and the request is accepted, a cell sends an RRC connection re-establishment message to the UE.

The UE that has received the RRC connection re-establishment message from the cell reconfigures a PDCP sublayer and an RLC sublayer with an SRB1. Furthermore, the UE calculates various key values related to security setting, and reconfigures a PDCP sublayer responsible for security as the newly calculated security key values. Accordingly, the SRB 1 between the UE and the cell is open, and the UE and the cell may exchange RRC control messages. The UE completes the restart of the SRB1, and sends an RRC connection re-establishment complete message indicative of that the RRC connection re-establishment procedure has been completed to the cell (S760).

In contrast, if the RRC connection re-establishment request message is received from the UE and the request is not accepted, the cell sends an RRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfully performed, the cell and the UE perform an RRC connection reconfiguration procedure. Accordingly, the UE recovers the state prior to the execution of the RRC connection re-establishment procedure, and the continuity of service is guaranteed to the upmost.

FIG. 8 illustrates sub states where the terminal may have in an RRC IDLE state and a sub state transition process.

Referring to FIG. 8, a terminal performs an initial cell selection process (S801). The initial cell selection process may be performed when there is no stored cell information with respect to the PLMN or a suitable cell is not found.

If the suitable cell is not found in the initial cell selection process, the terminal transitions to an any cell selection state (S802). The optional cell selection state represents a state which does not camp on in both of a suitable cell and an acceptable cell. The optional cell selection state is a state attempted by the terminal in order to find an acceptable cell of an optional PLMN which may camp on. When the terminal finds no cells which may camp on, the terminal is continuously maintained in an optional cell selection state until the acceptable cell is found.

If the suitable cell is found in the initial cell selection process, the state transits to a normal camp state (S803). The normal camp state represents a state which camps on the normal cell. A paging channel is selected according to information given through system information to motor, and an evaluation process for cell reselection may be performed.

In the normal camp state (S803), if a cell reselection evaluation process (S804) is caused, the cell reselection evaluation process (S804) is performed. If a suitable cell is found in the cell reselection evaluation process (S804), the terminal again transits to the normal camp state (S803).

If an acceptable cell is found in the any cell selection state (S802), the terminal transits to an any cell camped state (S805). The any cell camped state (S805) represents a state of camping on an acceptable cell.

In the any cell camped state (S805), the terminal may select a paging channel according to information given through system information to monitor, and may perform a cell reselection evaluation process (S806). If the acceptable cell is not found in the cell reselection evaluation process (S806), the terminal transits the any cell selection state (S802).

Hereinafter, a D2D operation will be described. In the 3GPP LTE-A, a service related to the D2D operation refers to Proximity based Services (ProSe). Hereinafter, the ProSe is an equivalent concept with the D2D operation and the ProSe may be compatibly used with the D2D operation. The ProSe is now described.

The ProSe includes ProSe direct communication and ProSe direct discovery. The ProSe direct communication presents communication performed by two or more adjacent terminals. The terminals may perform communication using a protocol of a user plane. A ProSe-enabled UE means a UE for supporting a process related to requirements of the ProSe. Unless otherwise defined, the ProSe-enabled UE includes both of a public safety UE and a non-public safety UE. The public safety UE represents a UE for supporting both of a public safety specified function and the ProSe process. The non-public safety UE is a terminal which supports the ProSe process but does not support the public safety specified function.

The ProSe direct discovery is a process where the ProSe-enabled UE discovers another ProSe-enabled UE. In this case, only ability of the two ProSe-enabled UEs is used. An EPC-level ProSe discovery signifies a process where an EPC determines whether 2 ProSe enable terminals are closed to each other, and reports the close state thereof the two ProSe enabled terminals.

Hereinafter, the ProSe direct communication may refer to D2D communication, and the ProSe direct discovery may refer to D2D discovery.

FIG. 9 illustrates a reference structure for a ProSe.

Referring to FIG. 9, the reference structure for a ProSe includes a plurality of terminals having E-UTRAN, EPC, and ProSe application program, a ProSe application (APP) server, and a ProSe function.

An EPC is a representative example of the E-UTRAN. The EPC may include an MME, an S-GW, a P-GW, a policy and charging rules function (PCRF), and a home subscriber server (HSS).

The ProSe application server is a user of ProSe in order to make an application function. The ProSe application server may communicate with an application program in the terminal. The application program in the terminal may use a ProSe ability to make an application function.

The ProSe function may include at least one of following functions but is not limited thereto.

Interworking via a reference point towards the 3rd party applications

Authorization and configuration of the UE for discovery and direct communication)

Enable the function of the EPC level ProSe discovery

ProSe related new subscriber data and handling of data storage, and also handling of ProSe identities

Security related function

Provide control towards the EPC for policy related function

Provide function for charging (via or outside of EPC, e.g., offline charging))

Hereinafter, a reference point and a reference interface will be described in a reference structure for the ProSe.

PC1: a reference point between a ProSe application program in the terminal and a ProSe application program in a ProSe application server. The PC1 is used to define signaling requirements in an application level.

PC2: is a reference point between the ProSe application server and a ProSe function. The PC2 is used to define an interaction between the ProSe application server and a ProSe function. An application data update of a ProSe database of the ProSe function may be an example of the interaction.

PC3: is a reference point between the terminal and the ProSe function. The PC3 is used to define an interaction between the terminal and the ProSe function. Configuration for ProSe discovery and communication may be an example of the interaction.

PC4: is a reference point between an EPC and the ProSe function. The PC4 is used to define an interaction between the EPC and the ProSe function. The interaction lay illustrate when a path for 1:1 communication or a ProSe service for real time session management or mobility management are authorized.

PC5: is a reference point to use control/user plane for discovery, communication, and relay between terminals, and 1:1 communication.

PC6: is a reference point to use a function such as ProSe discovery between users included in different PLMNs.

SGi: may be used for application data and application level control information exchange.

<ProSe Direct Communication (D2D Communication)>.

The ProSe direct communication is a communication mode where two public safety terminals may perform direct communication through a PC 5 interface. The communication mode may be supported in both of a case of receiving a service in coverage of E-UTRAN or a case of separating the coverage of E-UTRAN.

FIG. 10 illustrates arrangement examples of terminals performing ProSe direct communication and cell coverage.

Referring to FIG. 10(a), UEs A and B may be located outside of the cell coverage. Referring to FIG. 10(b), the UE A may be located in the cell coverage and the UE B may be located outside of the cell coverage. Referring to FIG. 10(c), both of UEs A and B may be located in the cell coverage. Referring to FIG. 10(d), the UE A may be located in coverage of a first cell and the UE B may be in coverage of a second cell.

As described above, the ProSe direct communication may be performed between terminals which are provided at various positions.

Meanwhile, following IDs may be used in the ProSe direct communication.

Source layer-2 ID: The source layer-2 ID identifies a sender of a packet in a PC 5 interface.

Purpose layer-2 ID: The purpose layer-2 ID identifies a target of a packet in a PC 5 interface.

SA L1 ID: The SA L1 ID represents an in an ID in a scheduling assignment (SA) in the PC 5 interface.

FIG. 11 illustrates a user plane protocol stack for the ProSe direct communication.

Referring to FIG. 11, the PC 5 interface includes a PDCH layer, a RLC layer, a MAC layer, and a PHY layer.

There may not be HARQ feedback in the ProSe direct communication. An MAC header may include the source layer-2 ID and the purpose layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>.

A ProSe enable terminal may use following two modes with respect to resource assignments for the ProSe direct communication.

1. Mode 1

The mode 2 is a mode for receiving scheduling a resource for the ProSe direct communication from a base station. The terminal should be in a RRC_CONNECTED state according to the mode 1 in order to transmit data. The terminal requests a transmission resource to the base station, and the base station schedules a resource for scheduling assignment and data transmission. The terminal may transmit a scheduling request to the base station and may transmit a Buffer Status Report (ProSe BSR). The base station has data which the terminal will perform the ProSe direct communication and determines whether a resource for transmitting the data is required.

2. Mode 2

The mode 2 is a mode for selecting a direct resource. The terminal directly selects a resource for the ProSe direct communication from a resource pool. The resource pool may be configured by a network or may be previously determined.

Meanwhile, when the terminal includes a serving cell, that is, when the terminal is in an RRC_CONNECTED state with the base station or is located in a specific cell in an RRC_IDLE state, the terminal is regarded to be in coverage of the base station.

If the terminal is located outside of the coverage, only the mode 2 is applicable. If the terminal is located in the coverage, the mode 1 or the mode 2 may be used according to setting of the base station.

If there are no exceptional conditions, only when the base station is configured, the terminal may change a mode from the mode 1 to the mode 2 or from the mode 2 to the mode 1.

<ProSe Direct Discovery (D2D Discovery)>

The ProSe direct discovery represents a process used to discover when the ProSe enabled terminal discovers other neighboring ProSe enabled terminal and refers to D2D direction discovery or D2D discovery. In this case, an E-UTRA wireless signal through the PC 4 interface may be used. Hereinafter, information used for the ProSe direct discovery refers to discovery information.

FIG. 12 illustrates a PC 5 interface for D2D discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHY layer, and a ProSe Protocol layer being an upper layer. Permission for announcement and monitoring of discovery information is handled in the upper layer ProSe Protocol. Contents of discovery information are transparent to an access stratum (AS). The ProSe Protocol allows only valid discovery information to be transferred to the AS for announcement.

An MAC layer receives discovery information from the upper layer ProSe Protocol. An IP layer is not used for transmitting the discovery information. The MAC layer determines a resource used in order to announce the discovery information received from the upper layer. The MAC layer makes and sends a protocol data unit (MAC PDU) to a physical layer. An MAC header is not added.

There are two types of resource assignments for announcing the discovery information.

1. Type 1

The type 1 is a method assigned so that resources for announcing the discovery information are not terminal-specific and the base station provides resource pool configuration for announcing the discovery information to the terminals. The configuration may be included in a system information block (SIB) to be signaled in a broadcast scheme. Alternatively, the configuration may be included in a terminal specific RRC message to be provided. Alternatively, the configuration may be broadcast-signaled or terminal-specific signaled of a different layer from the RRC message.

The terminal selects a resource from an indicated resource pool to announce discovery information using the selected resource. The terminal may announce discovery information through a resource optionally selected during each discovery period.

2. Type 2

The type 2 is a method where resources for announcing the discovery information are terminal-specifically assigned. A terminal in a RRC_CONNECTED state may request a resource for announcing a discovery signal to the base station through a RRC signal. The base station may assign a resource for announcing a discovery signal as an RRC signal. A resource for monitoring the discovery signal in a configured resource pool may be assigned in terminals.

With respect to a terminal in an RRC_IDLE state, a base station may report a type 1 resource pool for announcing the discovery signal as an SIB. Terminals where ProSe direct discovery is allowed use a type 1 resource pool for announcing the discovery information in the RRC_IDLE state. Alternatively, the base station 2) reports that the base station supports the ProSe direct discovery through the SIB but may not provide the resource for announcing the discovery information. In this case, the terminal should enter the RRC_CONNECTED state for announcing the discovery information.

With respect to a terminal in an RRC_CONNECTED state, the base station may configure whether to use a type 1 resource pool or a type 2 resource pool for announcing the discovery information through a RRC signal.

Hereinafter, the present invention will be described.

As an example, a D2D operation may have various advantages in that the D2D operation is signal transmission/reception between neighboring devices. For example, a D2D terminal has high data rate and low latency and is capable of data communication. In addition, the D2D operation may distribute traffic which concentrates on a base station and can also serve to expand the coverage of the base station if a terminal performing the D2D operation serves as a relay.

By extension of the D2D communication, D2D communication related to a vehicle, which includes inter-vehicle signal transmission/reception is called, in particular, vehicle-to-X (V2X) communication. Herein, as an example, in vehicle-to-X (V2X), the term ‘X’ means PEDESTRIAN (COMMUNICATION BETWEEN A VEHICLE AND A DEVICE CARRIED BY AN INDIVIDUAL (e.g.,) HANDHELD TERMINAL CARRIED BY A PEDESTRIAN, CYCLIST, DRIVER OR PASSENGER)) (V2P), VEHICLE (COMMUNICATION BETWEEN VEHICLES) (V2V), INFRASTRUCTURE/NETWORK (COMMUNICATION BETWEEN A VEHICLE AND A ROADSIDE UNIT (RSU)/NETWORK (e.g.,) RSU IS A TRANSPORTATION INFRASTRUCTURE ENTITY (e.g.,) AN ENTITY TRANSMITTING SPEED NOTIFICATIONS) IMPLEMENTED IN AN eNB OR A STATIONARY UE)) (V2I/N), etc. As an example, for easy description of a proposal scheme, a device (related to the V2P communication) owned by a pedestrian (or a person) is referred to as “P-UE” and a device (related to the V2X communication) installed in the vehicle is referred to as “V-UE”. Further, as an example, in the present invention, the term ‘ENTITY’ may be construed as P-UE and/or V-UE and/or RSU (NETWORK/INFRASTRUCTURE).

In general, a terminal performs the V2X communication based on limited power. Accordingly, the terminal may perform transmission based on relatively high power for a signal having a relatively high priority and perform transmission based on relatively low power for a signal having a relatively low priority.

In this case, when transmission of the signal having the relatively low priority is performed, power lower than minimum power required for the transmission of the signal having the relatively low priority may be allocated (due to partial or complete) overlapping with transmission of a signal having a relatively high priority).

Accordingly, the present invention intends to provide a method for solving a problem that, when transmitting the signal with the relatively low priority, the power lower than the minimum power is allocated and an apparatus using the method. That is, the following proposal schemes present a method for guaranteeing an LP_V2XMSG transmission/reception operation with high reliability when transmission power sufficient for corresponding LP_V2XMSG transmission may not allocated (alternatively, when transmission power lower than required transmission power needs to be allocated or when transmission power lower than a predetermined (or signaled) threshold needs to be allocated) due to (partial or complete) overlapping of “transmission time point (a signal/channel having a relatively high priority (referred to as “HP_MSG”)” and “transmission time point of a V2X message (referred to as “LP_V2XMSG”) having a relative low priority”.

Moreover, hereinafter, for easy description, the specification is described mainly from the viewpoint of the V2X communication, but it is not intended to exclude that the embodiment is applied to D2D communication within the scope of the present invention. That is, hereinafter, the V2X communication may be replaced with the D2D communication and the V2X terminal may be replaced with the D2D terminal.

FIG. 13 is a flowchart of a method for performing V2X communication according to an embodiment of the present invention.

Referring to FIG. 13, the UE may determine whether a transmission time point of a first message having a high priority overlaps with the transmission time point of a second message having a relatively low priority than the first message (S1310). In this case, the first message may mean the HP_MSG and the second message may mean the LP_V2XMSG.

As an example, in the present invention, a situation that the HP_MSG(S) and the LP_V2XMSG(S) are transmitted over different carrier(s) (and/or the HP_MSG(S) and the LP_V2XMSG(S) are transmitted through different frequency resources on the same carrier) is assumed.

That is, the overlapping of the transmission time points of the high priority message (e.g., HP_MSG) and the low priority message (e.g., LP_V2XMSG) means that the transmission time point of the high priority message and the transmission time point of the low priority message overlap in an FDM format in one carrier FDM. Moreover, the overlapping of the transmission time points of the high priority message (e.g., HP_MSG) and the low priority message (e.g., LP_V2XMSG) means that the transmission time point of the high priority message and the transmission time point of the low priority message overlap in different carriers.

As an example, the HP_MSG(S) may be specified as some of the following signals/channels/messages.

(1) e.g.,) ‘WAN UL SIGNAL/CHANNEL’ (e.g.,) PUCCH, PUSCH).

Ex) ‘WAN UL SIGNAL/CHANNEL’ transmitted in CARRIER # A may be construed as having a higher priority (in terms of transmission power allocation) than ‘V2X MESSAGE’ transmitted in CARRIER # B. In summary, in this example, ‘WAN UL SIGNAL/CHANNEL’ may correspond to the high priority message and ‘V2X MESSAGE’ may correspond to the low priority message.

(2) Ex) ‘V2X SYNCHRONIZATION SIGNAL’ (and/or V2X COMMUNICATION CHANNEL and/or V2X DISCOVERY CHANNEL)

Ex) ‘V2X SYNCHRONIZATION SIGNAL’ transmitted in CARRIER # A may be construed as having a higher priority (in terms of transmission power allocation) than ‘V2X COMMUNICATION CHANNEL (and/or V2X DISCOVERY CHANNEL)’ transmitted in CARRIER # B. In summary, in this example, ‘V2X SYNCHRONIZATION SIGNAL’ may correspond to the high priority message and ‘V2X COMMUNICATION CHANNEL (and/or V2X DISCOVERY CHANNEL)’ may correspond to the low priority message.

(3) Ex) ‘V2X SIGNAL/CHANNEL ON SERVING CARRIER’

Ex) ‘V2X MESSAGE’ transmitted in SERVING CARRIER # A may be construed as having a higher priority (in terms of transmission power allocation) than ‘V2X MESSAGE’ transmitted in NON-SERVING CARRIER # B. In summary, in this example, ‘V2X MESSAGE’ transmitted in SERVING CARRIER # A may correspond to the high priority message and ‘V2X MESSAGE’ may correspond to the low priority message transmitted in NON-SERVING CARRIER # B.

Thereafter, when the transmission time of the first message having a higher priority overlaps with the transmission time point of the second message having the lower priority, transmission of the second message may be performed using the transmission power lower than the transmission power of the first message (S1320). Herein, the transmission of the second message may be (basically) configured to be repeated a predetermined number of times and the predetermined number of times may have a value of N (where N is a natural number).

Detailed methods for guaranteeing the LP_V2XMSG transmission/reception operation with high reliability when the transmission power sufficient for corresponding LP_V2XMSG transmission may not allocated (alternatively, when the transmission power lower than required transmission power needs to be allocated or when the transmission power lower than a predetermined (or signaled) threshold needs to be allocated) (reduction of LP_V2XMSG coverage/performance) due to (partial or complete) overlapping of “the transmission time point of HP_MSG” and “the transmission time point of LP_V2XMSG”. Herein, as an example, transmission of LP_V2XMSG may be basically repeated ‘N’ times as described above.

Hereinafter, in regard to a specific example of a method for performing the second message transmission at a predetermined number of times when the transmission time point of the first message having the high priority and the transmission time point of the second message having the low priority overlap with each other, (1) a method for transmitting the second message at a number of times greater than the predetermined number of transmission times of the second message when the transmission time point of the first message overlaps with the transmission time point of the second message and (2) a method for applying transmission power having a relatively larger value to transmission of the second message at a remaining number of times (e.g., ‘(N-Q)’) excluding the number of times (e.g., ‘Q’, Q is the natural number, Q<N) of transmission of the second message in which power reduction occurs from the predetermined number of times (e.g., ‘N’, N is a natural number of 2 or more) when the transmission time point of the first message overlaps with the transmission time point of the second message will be described with reference to the drawings.

1. Method for transmitting the second message at a number of times greater than the predetermined number of transmission times of the second message when the transmission time point of the first message overlaps with the transmission time point of the second message

First, when the transmission power of the second message decreases due to overlapping of the transmission time point of the message having the high priority overlaps with the transmission time point of the message having the low priority, the UE may increase the number of repeated transmission times of the second message compared with a predetermined value (e.g., ‘N’ times). Accordingly, the UE may compensate for the LP_V2XMSG coverage/performance reduction due to the decrease in transmission power by repeated transmission. Hereinafter, the method will be described through the drawing.

FIG. 14 is a flowchart of a method for performing V2X communication according to another embodiment of the present invention.

Referring to FIG. 14, the UE may determine whether a transmission time point of a first message having a high priority overlaps with the transmission time point of a second message having a relatively lower priority than the first message (S1410). A specific example in which the UE determines whether the transmission time point of the first message having the high priority and the transmission time point of the second message having the low priority overlap with each other is described above. The concrete contents of the first message and the second message are also described above. For example, as described above, the first message having the high priority may mean (1) ‘WAN UL SIGNAL/CHANNEL’ (e.g., PUCCH and PUSCH), (2) ‘V2X SYNCHRONIZATION SIGNAL’ (and/or V2X COMMUNICATION CHANNEL and/or V2X DISCOVERY CHANNEL), or (3) ‘V2X SIGNAL/CHANNEL ON SERVING CARRIER’. In addition, as described above, in the case of the second message having the relatively low priority is (1) when ‘WAN UL SIGNAL/CHANNEL’ is the high priority message, ‘V2X MESSAGE’ may correspond to the low priority message, (2) when ‘V2X SYNCHRONIZATION SIGNAL’ is the high priority message, ‘V2X COMMUNICATION CHANNEL (and/or V2X DISCOVERY CHANNEL)’ may correspond to the low priority message, or (3) when ‘V2X MESSAGE’ transmitted from SERVING CARRIER # A is the high priority message, ‘V2X MESSAGE’ transmitted from NON-SERVING CARRIER # B may correspond to the low priority message.

When the transmission time point of the first message having the higher priority overlaps with the transmission time point of the second message having the lower priority, the UE may transmit at the number of times (e.g., K is a natural number having a value greater than N) than a predetermined number of times (e.g., N, N is the natural number) (S1420). More specifically,

[Proposal Method #1] The LP_V2XMSG transmission may be configured to be performed at the predefined (signaled) number of repeated transmission times having a relatively large value (K).

Herein, as an example, a relationship of ‘K>N’ is established.

As another example, different K values may be configured (or signaled) to be applied according to a degree to which the transmission power of LP_V2XMSG is lowered (relative to required transmission power or relative to a predefined (or signaled) threshold).

Herein, as an example, when the transmission power reduction of LP_V2XMSG corresponds to a range of ‘1 DB (DECIBEL)≤POWER REDUCTION VALUE (PRV)<0 DB’, ‘2 DB≤PRV<1 DB’, and ‘PRV<2 DB’, the numbers of repeated transmissions of ‘K=2’, ‘K=3’, and ‘K=4’ are configured (or signaled) (assuming ‘N=1’) to be applied to ‘1 DB (DECIBEL) POWER REDUCTION VALUE (PRV)<0 DB’, ‘2 DB≤PRV<1 DB’, and ‘PRV<2 DB’, respectively. Here, as an example, such a rule may be construed as applying a relatively larger number of repeated transmission times as the LP_V2XMSG transmission power reduction becomes larger (LP_V2XMSG coverage/performance reduction due to transmission power reduction is compensated by repeated transmission).

As another example, in particular, when the transmission power reduction of LP_V2XMSG is not temporary, “power values that are not actually used relative to the existing target (power) are accumulated” and different K values may be configured (or signaled) to be applied according to the accumulated values.

As yet another example, since increasing the number of repeated transmission times (in terms of LP_V2XMSG performance) (in accordance with the methods described above) when a load (or congestion) state is high may cause a larger problem, ‘whether an operation of increasing the number of repeated transmission times is applied’ may be configured through predefined signaling or configured to be applied (limitedly) only when the load (or congestion) state determined by a V2X TX entity is lower than (a predetermined (or signaled) threshold).

As still yet another example, when the V2X TX entity increases the number of repeated transmission times (according to the methods described above), information on the corresponding number of repeated transmission times may be configured to be transmitted while being included (embedded) in LP_V2XMSG.

In the present invention, information on the load (or congestion) state may be determined based on information directly measured by the V2X TX UE. Alternatively, after a base station or RX UE measures the information on the load (or congestion) state, the base station or the RX UE may transmit (or notify) the information on the load (or congestion) state to the V2X TX UE through predefined signaling.

2. Method for applying transmission power having a relatively larger value to transmission of the second message at a remaining number of times (e.g., ‘(N-Q)’) excluding the number of times (e.g., ‘Q’, Q is the natural number, Q<N) of transmission of the second message in which power reduction occurs from the predetermined number of times (e.g., ‘N’, N is a natural number of 2 or more) when the transmission time point of the first message overlaps with the transmission time point of the second message

FIG. 15 is a flowchart of a method for performing V2X communication according to yet another embodiment of the present invention.

Referring to FIG. 15, the UE may determine whether a transmission time point of a first message having a high priority overlaps with the transmission time point of a second message having a relatively low priority (S1510). Moreover, a specific example in which the UE determines whether the transmission time point of the first message having the high priority and the transmission time point of the second message having the low priority overlap with each other and a specific example of the first message having the high priority and the second message having the low priority are described above. For example, as described above, the first message having the high priority may mean (1) ‘WAN UL SIGNAL/CHANNEL’ (e.g., PUCCH and PUSCH), (2) ‘V2X SYNCHRONIZATION SIGNAL’ (and/or V2X COMMUNICATION CHANNEL and/or V2X DISCOVERY CHANNEL), or (3) ‘V2X SIGNAL/CHANNEL ON SERVING CARRIER’. In addition, as described above, in the case of the second message having the relatively low priority is (1) when ‘WAN UL SIGNAL/CHANNEL’ is the high priority message, ‘V2X MESSAGE’ may correspond to the low priority message, (2) when ‘V2X SYNCHRONIZATION SIGNAL’ is the high priority message, ‘V2X COMMUNICATION CHANNEL (and/or V2X DISCOVERY CHANNEL)’ may correspond to the low priority message, or (3) when ‘V2X MESSAGE’ transmitted from SERVING CARRIER # A is the high priority message, ‘V2X MESSAGE’ transmitted from NON-SERVING CARRIER # B may correspond to the low priority message.

Thereafter, the UE performs transmission of the second message corresponding to the remaining number of times (e.g., ‘(N-Q)’) excluding the number of times (e.g., ‘Q’, Q is the natural number, Q<N) of transmission of the second message in which power reduction occurs from the predetermined number of times (e.g., ‘N’, N is a natural number of 2 or more) by using the transmission power having the relatively larger value when the transmission time point of the first message having the high priority overlaps with the transmission time point of the second message having the low priority (S1520). More specifically,

[Proposal Method #2] Predefined (or signaled) transmission power (or an additional transmit power offset value) having the relatively large value may be configured to be applied to remaining (‘(N-Q)’) transmissions other than ‘Q’ times where the transmission power reduction actually occurs Among the LP_V2XMSG transmissions at ‘N’ times.

Herein, as an example, the V2X TX entity is configured to be allowed to assume the additional transmission power offset value as a value obtained by dividing “the accumulated power value that may not be actually used relative to the existing target (power) (or required power)” by ‘(N-Q)’.

Herein, as an example, such a rule may be construed to compensate for the LP_V2XMSG coverage/performance reduction due to the decrease in transmission power, which occurs in the LP_V2XMSG transmissions at ‘Q’ times by increasing the transmission powers of the remaining (‘N-Q’) LP_V2XMSG transmissions.

Further, as another example, increasing the power with respect to a feature message may cause interference in other messages. Accordingly, the LP_V2XMSG transmissions with increased transmission power may be configured to be performed in predefined (or signaled) independent ‘RESOURCE POOL’ of the corresponding use by considering interference (e.g., IN-BAND EMISSION) on different V2X message transmission and/or WAN uplink signal/channel transmission (in accordance with the methods described above).

Further, as an example, when the V2X TX entity increases the transmission power (according to the methods described above), information on the corresponding increased transmission power may be configured to be transmitted while being included (embedded) in LP_V2XMSG. Accordingly, performance of modulation may be enhanced.

The following embodiments may be applied in parallel with or in combination with the above embodiments.

[Proposal Method #3] When V2X TX entity(s) is allowed to select ‘TX resource(s)’ related to LP_V2XMSG transmission,

(A) ‘TX resource(s)’ related to LP V2XMSG transmission may be configured to be selected among remaining ‘TX resource(s)’ other than TX resource(s)' in which LP_V2XMSG transmission needs to be with transmission power lower than the (existing) required transmission power (or a predefined (or signaled) threshold) on the predefined (or signaled) ‘RESOURCE POOL’.

For example, when the UE needs to select four transmission resources in order to transmit the message having the relatively low priority, the UE may select four transmission resources among the transmission resources other than the transmission resources in which LP_V2XMSG transmission needs to be performed with the transmission power lower than the required transmission power (or predefined (or signaled) threshold).

Alternatively, (B) once the UE selects ‘TX resource(s)’ related to LP_V2XMSG transmission on the predefined (or signaled) ‘RESOURCE POOL’ when the UE needs to perform LP_V2XMSG transmission with the transmission power lower than the (existing) required transmission power (or predefined (or signaled) threshold) in the corresponding selected ‘TX resource(s)’, the UE may be configured to reselect ‘TX resource(s)’ related to LP_V2XMSG transmission.

For example, when the UE needs to select four transmission resources to transmit the message with the relatively low priority, the UE first selects four transmission resources and when there is a transmission resource in which LP_V2XMSG transmission needs to be performed with the transmission power lower than the (existing) required transmission power (or predefined (or signaled) threshold), the UE may reselect the transmission resource to replace the transmission resource in which LP_V2XMSG transmission needs to be performed with the low transmission power.

Here, as an example, in the latter case (B), the V2X TX entity(s) repeats a (re)selection process of ‘TX resource(s)’ related to LP_V2XMSG transmission until LP_V2XMSG transmission is not performed with the transmission power lower than the (existing) required transmission power (or predefined (or signaled) threshold) in the selected ‘TX resource(s)’.

It is obvious that since the examples of the proposed method described above may also be included as one of implementing methods of the present invention, the examples may be regarded as a kind of proposed methods. Further, the proposed methods described above may be independently implemented, but implemented in combination (or merge) of some of the proposed methods. For example, in the present invention, the proposed method based on the 3GPP LTE system has been described for easy description, but the scope of the system to which the proposed method is applied may be extended to other systems than the 3GPP LTE system. As an example, the proposed methods of the present invention may be extended even for D2D communication. Herein, as an example, the D2D communication means that a UE communicates directly with another UE using a wireless channel. Herein, as an example, the UE refers to a user's terminal, but a network equipment such as a base station may also be regarded as a kind of UE when transmitting/receiving the signal according to a communication scheme between the UEs.

FIG. 16 is a block diagram illustrating a UE in which the embodiment of the present invention is implemented.

Referring to FIG. 16, the UE 1100 includes a processor 1110, a memory 1120, and a radio frequency (RF) unit 1130. The processor 1110 may determine whether a transmission time point of a first message having a high priority overlaps with the transmission time point of a second message having a relatively lower priority than the first message. Further, the processor 1110 may perform the second message at a predetermined number of times when the transmission time point of the first message having the high priority and the transmission time point of the second message having the low priority overlap with each other.

The RF unit 1130 is connected with the processor 1110 to transmit and receive a radio signal.

The processor may include an application-specific integrated circuit (ASIC), another chipset, a logic circuit and/or a data processing apparatus. The memory may include a read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium, and/or other storage devices. The RF unit may include a baseband circuit for processing the radio signal. When the embodiment is implemented by software, the aforementioned technique may be implemented by a module (a process, a function, and the like) that performs the aforementioned function. The module may be stored in the memory and executed by the processor. The memory may be positioned inside or outside the processor and connected with the processor by various well-known means. 

What is claimed is:
 1. A method for device-to-device (D2D) operation in a wireless communication system, the method performed by a user equipment (UE) and comprising: determining whether a transmission time point of a first message having a high priority overlaps a transmission time point of a second message having a low priority; performing a transmission of the second message based on transmission power lower than transmission power of the first message, based on the determining, wherein the transmission of the second message is configured to repeat a predetermined number of transmission times, wherein, when the transmission time point of the first message overlaps the transmission time point of the second message, the second message is transmitted at a greater number of transmission times than the predetermined number of transmission times, wherein a value of the greater number of transmission times varies depending on a degree at which a value of the transmission power allocated to the transmission of the second message decreases, and wherein the value of the greater number of transmission times increases as the value of the transmission power allocated to the transmission of the second message decreases.
 2. The method of claim 1, wherein when a decrease in value of the transmission power allocated to the transmission of the second message is not temporary, differences between an existing value of the transmission power allocated to the transmission of the second message and other values of the transmission power allocated to the transmission of the second message are accumulated and the value of the greater number of transmission times varies depending on the accumulated values.
 3. The method of claim 1, further comprising: transmitting, to the base station, information informing that the second message is transmitted at the greater number of transmission times than the predetermined number of transmission times.
 4. The method of claim 1, wherein transmission power having a relatively large value is used for the transmission of the second message, which relates to a remaining number of transmission times other than the greater number of transmission times of the second message using the transmission power lower than the transmission power of the first message in the predetermined number of transmission times, when the transmission time point of the first message overlaps with the transmission time point of the second message.
 5. The method of claim 4, wherein a predetermined offset is applied to the transmission of the second message, which relates to the remaining number of transmission times.
 6. The method of claim 4, wherein the transmission of the second message, which relates to the remaining number of transmission times is performed in a dedicated resource pool.
 7. The method of claim 1, wherein the first message is a wide area network (WAN) uplink message and the second message is a D2D message.
 8. The method of claim 1, wherein the first message is a D2D synchronization message and the second message is a D2D communication message.
 9. The method of claim 1, wherein when the transmission time point of the first message overlaps the transmission time point of the second message, the transmission time point of the first message and the transmission time point of the second message overlap with each other in a frequency division multiplexing (FDM) form in one carrier or the transmission time point of the first message and the transmission time point of the second message overlap with each other in different carriers.
 10. The method of claim 1, wherein the D2D operation is a vehicle to X (V2X) operation.
 11. A user equipment (UE) comprising: a radio frequency (RF) transceiver transmitting and receiving a radio signal; and a processor operated in association with the RF transceiver, wherein the processor is configured to: determine whether a transmission time point of a first message having a high priority overlaps a transmission time point of a second message having a low priority; and perform a transmission of the second message based on transmission power lower than transmission power of the first message, based on the determine, wherein the transmission of the second message is configured to repeat a predetermined number of transmission times, wherein, when the transmission time point of the first message overlaps the transmission time point of the second message, the second message is transmitted at a greater number of transmission times than the predetermined number of transmission times, wherein a value of the greater number of transmission times varies depending on a degree at which a value of the transmission power allocated to the transmission of the second message decreases, and wherein the value of the greater number of transmission times increases as the value of the transmission power allocated to the transmission of the second message decreases. 